In oil and gas well operations, particularly in subsea applications, wellheads are used to support concentric casing strings which extend to varying depths in the wellbore. Landing shoulders are provided in such wellheads for support of one or more of these strings. These wellheads have seal bores and the landing shoulders provided in them generally define the maximum internal clearance so as to limit the maximum size of casing which can fit through the wellhead. That is to say, the seal bore has a greater diameter than the bore defined by the landing shoulder. The projecting landing shoulder limits the largest casing size that can be run through the wellhead. If it was not in the way, another larger casing string could fit through the same wellhead body. This is the objective of the invention, i.e., to get the landing shoulder out of the way so that a larger additional string can be run and later actuate the landing shoulder into the bore to get a support for the subsequent smaller casing strings.
The need to fit in an extra-large string into an existing wellhead configuration has arisen from subsea completions where substantial amounts of water are produced at very shallow depths into the wellbore. Previous techniques that have been available to deal with the infiltration of water into the wellbore at shallow depths have had undesirable costs associated with them. Since the water migration into the wellbore is at a fairly low depth, control with mud becomes costly since the mud is lost during the casing-running operation through the zones where water is infiltrating. Attempts to cement across the zone where water is infiltrating also involve significant costs with the amount of cement that is required to effectively accomplish the procedure. Thus, the need has arisen for a more effective technique to isolate the zone where water is infiltrating by running an additional string through an existing wellhead. For example, weighted mud can be used and recovered if 18" casing can be run through an 183/4" high-pressure housing. The problem has been that the minimum clearance in 183/4" housings is presently too small to accept 18" casing. Thus, one of the objectives of the present invention is to provide a wellhead housing with a remotely actuable landing shoulder which is mounted in a recess sufficient to allow very-close-clearance casing to be run through a high-pressure housing of a wellhead. At the same time, after the large casing is installed, it is another objective of the invention to be able to remotely actuate a landing shoulder to accept a smaller size casing which is run through the larger casing. It is desirable to actuate the landing shoulder remotely in a manner that gives a signal at the surface that the actuation has occurred and that the landing shoulder is in place. It is another objective of the present invention to configure the remotely actuable landing shoulder so that it cannot be accidentally actuated while the larger size casing is being run through the wellhead housing.
Designs in the past have employed either fixed landing shoulders in wellhead housings or shoulders that can be run-in with a casing string. These devices have generally had to catch recesses in the wellhead housing. The devices were generally held in place with shear pins and were prone to premature set. Operators at the surface did not have good feedback as to whether the string had set on the landing shoulder properly, and many times shoulders run-in with the string to catch a groove in the wellhead body would fail to actuate or fail to actuate in the proper location, undermining the desired intent of support of the string. A few prior designs are illustrated in U.S. Pat. Nos. 4,067,388; 4,917,191; 5,655,606. Also of general interest in the area of multi-string casing supports in wellheads is the STC-10 wellhead made by Cooper Cameron.